In recent years, with the advancement of the Ethernet (registered trademark) and IP (Internet Protocol) technologies, networks are rapidly becoming IP-based. This trend has been popular among network providers, so that the SDH (Synchronous Digital Hierarchy) transmission method is starting to be replaced with the packet transmission method, to address the increased demand for IP traffic of carrier networks and to enhance transmission efficiency for the purpose of reducing cost. The following describes the difference between the SDH transmission method and the packet transmission method in terms of transmission efficiency.
The SDH method is a technology based on TDM (time-division multiplex), which means time slots may be occupied (used) when there are no data to be transmitted. On the other hand, in the packet transmission method, when there is no data to be transmitted, the time slots may be used by another service; therefore the line (use) efficiency is improved.
In a carrier network, from the viewpoint of operational management, even when a packet transmission method is used, path control corresponding to the SDH method (i.e., static path setting) is to be performed. To meet this demand, a packet-based transport method called MPLS-TP (Multi Protocol Label Switching-Transport Profile) has been developed.
FIG. 1 indicates an example of a ring network, in which nodes #1 through #8 of the MPLS-TP scheme are connected in a ring shape by links #A through #H. FIG. 2 indicates a state where an LSP (Label Switch Path) is set in the network of FIG. 1, extending between node #1 and node #5 via nodes #2 through #4.
A carrier network is demanded to have high availability due to its nature. Actually, as indicated in FIGS. 3 and 4, there is a working LSP and a backup LSP. In FIG. 3, there is a working LSP in the A direction (clockwise direction) indicated by an arrow of a solid line, and a backup LSP in the B direction (counterclockwise direction) indicated by an arrow of a dashed line. At a receiving node (#5), signals of either the working LSP or the backup LSP are selected. Typically, a receiving node defining the terminal point is used for monitoring reception of a CCM (Continuity Check Message) packet of OAM (Operation, Administration, and Maintenance), which is a monitor control packet for a working LSP and a backup LSP, to perform fault recovery in units of LSP.
In FIG. 4, there is a ring-shaped working LSP in a clockwise direction indicated by an arrow of a solid line, and a ring-shaped backup LSP in a counterclockwise direction indicated by an arrow of a dashed line. When a fault occurs, the working LSP is connected to the backup LSP by a fault detection node, and fault recovery is performed by bypassing the fault section. In the fault recovery method of FIG. 4, signals do not flow into the backup LSP when a fault has not occurred, and therefore the line efficiency is higher than that of the fault recovery method of FIG. 3.
FIGS. 3 and 4 illustrate a fault recovery method of an LSP having a point to point configuration. However, there is demand for communication for performing multicasting to end users in the form of multimedia applications such as video streaming in a metro network and an internet protocol television (IPTV). Furthermore, in a cloud service, it is anticipated that demand will increase for 1:N connections, i.e., multicast communication. There are discussions of standardizing the recovery method of multicast communication at IETF, and various methods are being proposed (see, for example, non-patent document 1).
Incidentally, the following technology is proposed. There are multipoint logic paths not only for transmitting a frame transmitted from a transmitting terminal node to a multicast frame receiving terminal node, but also for transferring a frame to a transmitting terminal node by circulating a ring so as to terminate the frame also in the transmission terminal node. These multipoint logic paths are previously prepared for two routes of a working system and a backup system. A node which detects a fault transmits a forward fault notification frame to the multicast logic path where the fault has occurred. The transmission terminal node that has received the forward fault notification frame stops the use of the received multicast logic path and transmits the frame by a path which has not yet received the forward fault notification frame (see, for example, patent document 1).
Furthermore, the following technology is proposed. Along two or more paths in a network, communication signals are transmitted from ingress nodes to plural egress nodes. There is provided a primary path used for transmitting multicast communication signals along a communication link before a fault occurs in the network. There is also provided a backup path for transmitting multicast communication signals along a communication link. When a fault occurs, multicast communication signals are transmitted along the primary path at the same time as transmitting copies of the multicast communication signals along the backup path (see, for example, patent document 2).
Furthermore, the following technology is proposed. The node device operating as a working ring node or a backup ring node on the ring network is provided with a learning part for snooping and learning a message for performing data relay as data relay information before a fault occurs in the working ring node when the node device itself operates as a backup ring node, and a working/backup switching part for switching the node device from a backup ring node to a working ring node on the basis of the data relay information learned by the learning part when a fault occurs in a working ring node, in the case that the node device is operating as a backup ring node (see, for example, patent document 3).    Non-patent document 1: draft-liu-mpls-tp-ring-protection-01.txt    Non-patent document 2: draft-umansky-mpls-tp-ring-protection-switching-03.txt    Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-282153    Patent Document 2: Japanese National Publication of International Patent Application No. 2010-515314    Patent Document 3: Japanese Laid-Open Patent Publication No. 2007-228293
FIG. 5 indicates a configuration of an LSP in conventional multicast communication. A ring-shaped working LSP in a clockwise direction indicated by an arrow of a solid line is used for delivering packets from a transmitting node #1 to nodes #2, #3, #4, #6, #7, #8.
In a fault recovery method indicated in FIG. 6, when a fault occurs, the working LSP is connected to the ring-shaped backup LSP in a counterclockwise direction indicated by an arrow of a dashed line, at a fault detection node #6. Accordingly, the fault section is bypassed so that fault recovery is performed. In a fault recovery method indicated in FIG. 7, a fault at link #F detected by a node #6 and a node #7 is reported to the transmitting node #1, and the signals are bridged to both the working LSP and the backup LSP at the transmitting node #1, so that fault recovery is performed.
In the fault recovery methods indicated in FIGS. 6 and 7, it is assumed that the signals indicated in FIG. 5 are multicast in one direction (clockwise direction) under normal circumstances. Accordingly, under normal circumstances indicated in FIG. 5, even though the node #8 is adjacent to the transmitting node #1 in terms of the ring topology, the path circulates the ring, and therefore a large transmission delay is caused.
Furthermore, when request signals and response signals that are delivered by multicast from terminals and devices are transferred to the terminals and devices that are the multicast delivery sources, the nodes #8, #7, #6, #4, #3, #2 of FIG. 5 are set as transmitting nodes, and the signals that are added (inserted) at the respective nodes #8, #7, #6, #4, #3, #2 are transmitted to the node #1 acting as a receiving node. In this case also, the path circulates the ring, and therefore a large transmission delay is caused.